Head-up displays (HUDs) are becoming increasingly popular in the aerospace industry. Known HUDs typically include at least a projector, a combiner, and an image generator. The projector receives images supplied from the image generator, and the HUD will typically include an optical collimator, such as a convex lens or concave mirror, to produce an image that is perceived to be at infinity or other far distance.
The combiner reflects the image projected by the projector in such a way as to see the field of view and the projected infinity image at the same time. The combiner is typically a very precisely designed and controlled optical element and may be flat or curved. Some combiners may also have special coatings that reflect certain wavelengths of light projected onto it from the projector while allowing other wavelengths of light to pass through.
Traditional prior art HUDs typically rely on sophisticated optics to meet the performance requirements for avionic use. These performance requirements include precise angular control and uniformity over an exit pupil or head box that is large enough to encompass both eyes of a pilot or other user. As an example, the size, weight and cost of a bulky overhead unit (OHU) may be driven to a large extent by the required performance levels.
Referring to FIG. 1, a known HUD 100, for example, as included in an aircraft, includes a projector 102, and a combiner 104, for example a partially reflective element proximate the windscreen of the aircraft, that directs images 113 and 116 to a receiver 106, for example, a person's eyes. In the traditional aircraft HUD, the combiner 104 is typically a very precisely designed and controlled separate optical element, and includes a front, or inner, surface 108, and a back, or outer, surface 110. The projector 102 projects an image 112 onto the front surface 108, wherein the desired image 113 is reflected back toward the receiver 106 and some of the image 112 propagates through the combiner 104, reflecting off of the back surface 110 as a potentially undesired image 116. The images 113 and 116 can present a double image to the receiver 106 unless measures are taken to either reduce the reflectivity of back surface 110 relative to front surface 108 or to effectively align the two reflected images. The outside view 118 enters the combiner 104 at the back surface 110 and propagates through the combiner 104 to the receiver 106 combined with the reflected HUD images 113 and 116. Whereas combiner 104 has been described in this example as an optical element that is separate from and proximate the windscreen or windshield, certain vehicular HUD configurations, most notably automotive HUDs, may utilize the windshield or windscreen as the combiner rather than using a separate element. These will also involve the possibility of multiple visible reflected images.
Very often, and particularly with curved combiners, carefully designed and deposited coatings may be applied to the front and back surfaces. There are multiple objectives in selecting these coatings. One typical goal is to provide adequate reflectance of the HUD imagery while at the same time maintaining high “see-through” transmittance of the forward scene. Partially reflecting coatings would generally be applied on one of the surfaces, most commonly the front surface. Other objectives may include maximizing luminance of the HUD reflection while minimizing color tinting for the forward scene. Yet another objective may be to minimize any undesired visual artifacts associated with reflections from the rear surface of the combiner. As an example, the combiner surface might have a narrow band reflective multilayer interference coating, and the rear surface might have an anti-reflection coating of some type. However, when utilizing a vehicle windshield or windscreen as a combiner, the practicality and effectiveness of such a rear coating could be affected, for example due to rain or other contaminant buildup.
Accordingly, it is desirable to provide a HUD that suppresses double images from the combiner by eliminating or reducing a reflection of the HUD display from the back surface of the combiner. Furthermore, other desirable features and characteristics of the exemplary embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.